Table 1.

Number of Subsequent Cases After the First m Cases in the Data and in the Baseline and Restricted Models

Preexisting cases in household, no.	Subsequent cases, no.	Subsequent cases in baseline model, expected no.	Subsequent cases in restricted model, expected no.	Protective effect
1	704	705.04	705.04	0
2	333	339.88	330.12	.03
3	155	170.79	156.96	.08
4	75	74.46	64.89	.17
5	30	46.09	32.10	.32
6	12	19.44	13.23	.35

NOTE. The baseline model assumption was that the probability of disease was independent of number of previous exposures; in the restricted model, the probability of disease was allowed to vary with the number of previous exposures. The protective effect of m exposures was calculated by comparing the baseline and restricted models.